1. Field of the Invention
This invention relates in general to electric submersible pumps. More specifically, this invention relates to submersible pumps that have an impeller configuration designed for fluids with a high gas content entrained within the fluids.
2. Description of the Prior Art
Centrifugal pumps have been used for pumping well fluids for many years. Centrifugal pumps are designed to handle fluids that are essentially all liquid. Free gas frequently gets entrained within well fluids that are required to be pumped. The free gas within the well fluids can cause trouble in centrifugal pumps. As long as the gas remains entrained within the fluid solution, then the pump behaves normally as if pumping a fluid that has a low density. However, the gas frequently separates from the liquids.
The performance of a centrifugal pump is considerably affected by the gas due to the separation of the liquid and gas phases within the fluid stream. Such problems include a reduction in the pump head, capacity, and efficiency of the pump as a result of the increased gas content within the well fluid. The pump starts producing lower than normal head as the gas-to-liquid ratio increases beyond a certain critical value, which is typically about 10-15% by volume. When the gas content gets too high, the gas blocks all fluid flow within the pump, which causes the pump to become xe2x80x9cgas locked.xe2x80x9d Separation of the liquid and gas in the pump stage causes slipping between the liquid and gas phases, which causes the pump to experience lower than normal head. Submersible pumps are generally selected by assuming that there is no slippage between the two phases or by correcting stage performance based upon actual field test data and past experience.
Many of the problems associated with two phase flow in centrifugal pumps would be eliminated if the wells could be produced with a submergence pressure above the bubble point pressure to keep any entrained gas in the solution at the pump. However, this is typically not possible. To help alleviate the problem, gases are usually separated from the other fluids prior to the pump intake to achieve maximum system efficiency, typically by installing a gas separator upstream of the pump. Problems still exist with using a separator upstream of a pump since it is necessary to determine the effect of the gas on the fluid volume in order to select the proper pump and separator. Many times, gas separators are not capable of removing enough gas to overcome the inherent limitations in centrifugal pumps.
A typical centrifugal pump impeller designed for gas containing liquids consists of a set of one-piece rotating vanes, situated between two disk type shrouds with a balance hole that extends into each of the flow passage channels formed by the shrouds and two vanes adjacent to each other. In liquid lifting practice, an average value of 25 degrees is considered normal for all vane discharge angles. The size of the balance holes have traditionally been approximately xe2x85x9xe2x80x3 (0.125xe2x80x3) through {fraction (3/16)}xe2x80x3 (0.1875xe2x80x3) in diameter for most pump designs. Deviations from the typical pump configurations have been attempted in an effort to minimize the detrimental effects of gaseous fluids on centrifugal pumps. However, even using these design changes in the impellers of the centrifugal pumps is not enough, there are still problems with pump efficiency, capacity, and head.
One such attempt to modify a conventional centrifugal pump impeller for pumping fluids containing a high percentage of free gas can be found in U.S. Pat. No. 5,628,616 issued to Lee. The Lee Patent teaches the use of balance and recirculation holes for pressure equalization and recirculation of the fluid around the impeller. However, the impeller in Lee can only handle fluids containing up to 35% vol. of free gas. Above this level of gas content, the Lee pump would still become gas locked.
A need exists for an ESP and method of pumping high gas containing fluids without causing a pump to become gas-locked and unable to pump the fluid. Ideally, such a system should be capable of being adapted to the specific applications and also be able to be used on existing equipment with minimal modification.
Centrifugal pumps impart energy to a fluid being pumped by accelerating the fluid through an impeller. This invention provides a novel method and apparatus for pumping well fluids with a high gaseous content by utilizing a centrifugal pump with an improved impeller design that is optimized for use in gaseous liquids. The improved impeller has a new vane design, which can be combined with high discharge angles and large balance holes.
This invention introduces an unconventional split-vane impeller design with increased vane exit angle and oversized balance holes. The improvements provide homogenization to the two-phase flow due to the split-vane design. Pump performance is optimized by increased vane exit angle, which is typically in the range of about 50 degrees to about 90 degrees. The oversized balance holes provide additional gas and liquid mixing.
The split-vane impeller comprises two portions, an inner radial member and an outer radial member, with each portion having a different radius of curvature. An inner edge of the inner radial member is offset from an outer edge of the outer radial member, without the inner edge of the inner radial member contacting the outer radial member. The inner edge of the inner radial member can lead or trail the outer edge of the outer radial member. The space between the inner and outer radial members allows for improved mixing of the well fluid to assist in homogenizing the gas in the liquid phase.
The impeller has a plurality of flow passages that are defined by a split-vane on one side and a next split-vane on the opposite side. Each flow passage comprises one balance hole. The balance hole has a diameter in a range of about 45% to about 100% of a distance that is measured from the inner edge of the inner radial member to the outer edge of the next inner radial member. This range for the balance hole diameter corresponds to a diameter of at least {fraction (7/32)}xe2x80x3 (0.2188xe2x80x3) and greater. The balance hole can be substantially tangential to the split-vanes.
A centrifugal pump containing the impeller with the split-vanes, high exit angles, and balance holes can be used as a charge pump for a traditional centrifugal pump. As an alternative, the impeller designed in accordance with the present invention can be used in one or more stages within a centrifugal pump that also has one or more conventionally designed impellers. The centrifugal pump of the present invention can be used as part of a well assembly. A gas separator can be installed upstream of the charge pump to reduce the amount of free gas in the system prior to pumping. Other variations of the present invention will be known to those skilled in the art and are to be considered within the scope of the present invention.